In 1990, Gunasekera S P et al. reported the isolation of a new polyhydroxylated lactone, (+)-discodermolide, from the deep-water Caribbean sponge Discodermia dissoluta (Gunasekera S P et al. J. Org. Chem. 1990, 55, 4912-4915 and J. Org. Chem. 1991, 56, 1346).

This compound has been revealed to be a potent antimitotic agent (Hung D T et al. Chem. Biol. 1996, 3, 287-293 and ter Haar E et al. Biochemistry 1996, 35, 243-250), possessing a mode of action similar to that of the clinically proven anticancer agent paclitaxel (Schiff P B et al. Nature 1979, 277, 665-667). Both natural products arrest the cell cycle at the M phase, promote microtubule formation, and have similar inhibitory effects against breast cancer carcinoma (IC50 of 2.4 nM and 2.1 nM, respectively).
On the other hand, some unusual linear dipeptides containing a N-acyl enamide functionality have been isolated from a mycobacteria belonging to the Chondromyces genus (Kunze B et al. J. Antibiot. 1994, 47, 881-886 and Jansen R et al. J. Org. Chem. 1999, 1085-1089). Specifically, these compounds are crocacins A, B, C and D and are a group of electron transport inhibitors.

Crocacins A-D moderately inhibit the growth of a few Gram-positive bacteria and are potent inhibitors of animal cell cultures and several yeasts and fungi. The most active is crocacin D which showed a MIC of 1.4 ng/mL against the fungus Saccharomyces cerevisiae and strong toxicity (IC50 of 0.06 mg/L) toward L929 mouse fibroblast cell culture.
Gudasheva et al. (Russian Journal of Bioorganic Chemistry, 2007, 44(4), 413-420, and Pharmaceutical Chemistry Journal, 2006, 40(7), 367-372) reported the design of dipeptide compounds based on the structure of the endogenous tetrapeptide cholescystokinin-4 (CCK-4). In this regard, it is disclosed that L-thryptophan derivatives exhibited anxiolytic properties and the D-thryptophan derivatives, anxiogenic properties. Two of the dipeptide compounds disclosed by Gudasheva et al. are the following:
and the following compounds were disclosed as intermediates in the synthesis of compounds R and U:

Cancer is a leading cause of death in animals and humans. Huge efforts have been and are still being undertaken in order to obtain an antitumor agent active and safe to be administered to patients suffering from a cancer. The problem to be solved by the present invention is to provide compounds that are useful in the treatment of cancer.